1. Field of the Invention
This invention relates to an improved electrical cable and process for making the subject cable having a low dielectric constant, and in particular, a flexible cable having one or more conductors having improved transmission line characteristics, improved crush resistance, and capable of mass termination.
2. Description of Prior Art
There already exists in the marketplace multiconductor flexible, mass terminable cables having transmission line characteristics such as controlled impedance, crosstalk, propagation delay, etc. It is well known that by lowering the effective dielectric constant of the cable by including air in the dielectric, the signal speed can be increased.
Providing porosity in a dielectric suitable for cables is known. Foamed polyethylene insulative materials are known from U.S. Pat. No. 3,529,340, where the foam coated conductors were placed in a sheath which is shrunk onto the foam covered conductors. Another patent is U.S. Pat. No. 4,680,423, disclosing a foam-type insulation such as polypropylene or polyethylene surrounding conductors, which foam covered conductors are then embedded within an insulating material such as polyvinyl chloride. The foamed insulation is said to contain a large percentage of air trapped within the material. The insulating material is used to hold the conductors in a parallel configuration and provides strength to the cable when subjected to compression.
Another patent describing a foamed insulative material for conductors includes U.S. Pat. No. 5,110,998, issued May 5, 1992 describing an ultramicrocellular foamed polymer structure formed from suitable polymers including the class of synthetic, crystalline and crystallizable, organic polymers, e.g. polyhydrocarbons such as linear polyethylene, polypropylene, stereo-regular polypropylenes or polystyrene, polyethers such as polyvinylidene fluoride, polyamides both aliphatic and aromatic, and the list goes on, but concludes the polymers should have a softening point of at least about 40.degree. C. This foamed material, because of the high degree of orientation of the closed polyhedral cells, contributes to the strength of the structures.
Further, W. L. Gore & Associates, Inc. sells cable made with "Gortex" dielectric films, a porous polytetrafluoroethylene (PTFE). Polytetrafluoroethylene is not a conventional thermoplastic and is not easily processed and is costly. Various patents have been assigned to W. L. Gore & Associates, Inc. of Newark, Delaware including U.S. Pat. Nos. 3,953,566 and 4,187,390 relating to the process for making a porous polytetrafluoroethylene polymer; 4,443,657 relates to the manufacture of a ribbon cable using two layers of polytetrafluoroethylene (PTFE) as insulation, and 4,866,212 relating to a coaxial electric cable formed of an expanded polytetrafluoroethylene.
High speed cables of the prior art generally utilize expanded PTFE dielectrics such as those sold by W. L. Gore & Associates, Inc. or foamed perfluoro polymers. Such cable structures have lower crush resistance as compared to solid dielectrics. This lower crush resistance results in reduced transmission line performance as a result of damage caused by normal routing or handling of cables made from these conventional dielectrics.
The lack of crush resistance of known dielectrics used for cable insulation, which contain large percentages of air voids, has long been a problem for use as high speed dielectrics. In U.S. Pat. No. 4,730,088 assigned to Junkosha Co., LTD., Japan, the solution for improving crush resistance was reinforcing expanded polytetrafluoroethylene (PTFE) by the use of a laser beam or a hot metal rod. The piercing of the soft insulation by the beam or rod caused a unique phenomenon to occur to the porous PTFE called sintering. In this case, the sintering causes the soft dielectric to form a solid skin of PTFE on the inside wall of the created hole. Since sintered PTFE has many times the structural strength of the unsintered porous dielectric, the cylinders so created function like beams to resist crushing forces. An alternate method disclosed, used heated rolls to put grooves in the surface of the insulation. The sole purpose of both methods is to increase the crush resistance of the insulation. Both solutions suffer from the creation of discontinuities in the dielectric which add to signal speed variation as the electrical fields encounter these discontinuities.
U.S. Pat. No. 4,443,657, assigned to W.L. Gore and Associates, Inc., demonstrates a means of bonding sheets of PTFE using a sintering process. The softness of the unsintered core dielectric forces the inventor to place a solid layer of insulation over the top of the unsintered core resulting in significant reductions in electrical performance of the finished cable due to the solid dielectric.
Because of the very high processing temperatures of traditional PTFE cables, cables made in ribbon format with polytetrafluoroetylene generally have silver plated or nickel plated conductors to avoid the oxidation of the conductors during processing. Use of either of these plated conductors causes significant cost increase. In addition, if nickel is used, difficulty in soldering to the conductors is encountered.
It should be noted that lamination and fusion of thermoplastic insulations to make ribbon cables has been taught in the prior art such as U.S. Pat. No. 3,523,844 assigned to David J. Crimmins, et. al. and U.S. Pat. No. 2,952,728 assigned to Kyohei Yokose, et. al. The Crimmins patent teaches lamination of solid dielectrics around variably spaced wires. This method will not work with air filled dielectrics without collapsing the air filled structure. Similarly, the Yokose patent teaches lamination of solid dielectrics around conductors. However, the tool or roller design employed will cause excessive melting and destruction of the fibril structure of the material in the present invention. Both of the methods employed in the prior art would not work with the materials presented herein. The process and materials of the present invention teach lamination without significant destruction or collapse of the air filled structure adjacent the conductors.
The prior art demonstrates that many attempts have been made to provide electrical cables with lower dielectric constant and/or fixed shield-wire spacing to improve electrical characteristics. The prior art cables, even the foamed materials, sacrifice durability and crush resistance to achieve lower dielectric constant and faster propagation velocities. U.S. Pat. No. 5,110,998, describes a foamed structure for use as an insulative material for individual conductors smaller than 1.27 mm and annular insulation thickness less than 0.51 mm. The insulative material is flash spun over a moving wire in air at ambient temperature and pressure or by an extrusion spinning method. The crush resistance of the material is described in column 3 lines 64 to column 4 line 9. The recovery rate is not considered sufficient to provide good electrical properties to signal wire and the material is not suitable for making ribbon cable.
The present invention provides a product having improved crush resistance over unsintered expanded polytetrafluoroethylene without the time consuming and expensive process of forming sintered cylinders or grooves in the dielectric as disclosed in U.S. Pat. No. 4,730,088 assigned to Junkosha Co., LTD, Japan.
The product of the present invention in addition to having the improved electrical properties at substantially reduced cost and with improved crush resistance, does not have the dielectric discontinuities associated with the formation of sintered shapes as with prior art. The process used to form this product also can be accomplished at substantially reduced temperatures permitting conductors to be used with or without plating which provides additional cost reduction. The unique crush resistant properties of the subject product result since the polymers employed to make the insulation do not have the uncharacteristic changes caused by sintering as with PTFE but rather have the improved properties immediately upon cooling thus eliminating the costly and time consuming sintering processes.
Prior expanded materials, have also lacked this characteristic, in part due to the necessity to employ polymer structures which are inherently soft or weak in their structural integrity.
The prior art demonstrates that many attempts have been made to provide electrical cables with lower dielectric constant to improve electrical characteristics and to provide crush resistance to high speed dielectrics.